It is known that carbon-carbon double bonds in polymers may be successfully hydrogenated by treating the polymer in an organic solution with hydrogen in the presence of a catalyst. Such processes can be selective in the double bonds which are hydrogenated so that, for example, the double bonds in aromatic or naphthenic groups are not hydrogenated and double or triple bonds between carbon and other atoms such as nitrogen or oxygen are not affected. This field of art contains many examples of catalysts suitable for such hydrogenations, including catalysts based on cobalt, nickel, rhodium, ruthenium, osmium, and palladium. The suitability of the catalyst depends on the extent of hydrogenation required, the rate of the hydrogenation reaction and the presence or absence of other groups, such as carboxyl and nitrile groups, in the polymers. There have been already many patents and patent applications filed in this area, such as U.S. Pat. No. 6,410,657, U.S. Pat. No. 6,020,439, U.S. Pat. No. 5,705,571, U.S. Pat. No. 5,057,581, and U.S. Pat. No. 3,454,644.
However, many diene-based polymers, -copolymers or -terpolymers are made by emulsion polymerization processes and they are in latex form when they are discharged from polymerization reactors. Therefore it is very desirable to directly hydrogenate a diene-based polymer in latex form and this approach is receiving increasing attention in the recent decade. Many efforts have been made to realize such a process.
So far significant attention has been paid to the hydrogenation of C═C bonds using hydrazine or a derivative of hydrazine as a reducing agent together with an oxidant like oxygen, air or hydrogenperoxide. The hydrogen source to saturate the C═C bonds is then generated in-situ as a result of the redox reactions in which diimide is also formed as intermediate.
In U.S. Pat. No. 4,452,950 the latex hydrogenation is performed using the hydrazine hydrate/hydrogen peroxide (or oxygen) redox system to produce diimide in situ. CuSO4 or FeSO4 is used as a catalyst.
U.S. Pat. No. 5,039,737, and U.S. Pat. No. 5,442,009 provide a more refined latex hydrogenation process which treats the hydrogenated latex with ozone to break the cross-linked polymer chains which form during or after the latex hydrogenation using the diimide approach.
U.S. Pat. No. 6,552,132 discloses that a compound can be added before, during or after the latex hydrogenation to break crosslinks formed during the hydrogenation using the diimide hydrogenation route. The compound can be chosen from primary or secondary amines, hydroxylamine, imines, azines, hydrazones and oximes.
U.S. Pat. No. 6,635,718 describes the process for hydrogenating C═C bonds of an unsaturated polymer in the form of an aqueous dispersion by using hydrazine and an oxidizing compound in the presence of a metal compound containing a metal atom in an oxidation state of at least 4 (such as Ti(IV), V(V), Mo(VI) and W(VI)) as the catalyst.
In Applied Catalysis A-General Vol. 276, no. 1-2, 2004, 123-128 and Journal of Applied Polymer Science Vol. 96, no. 4, 2005, 1122-1125 detailed investigations relating to the hydrogenation of nitrile butadiene rubber latex via utilization of the diimide hydrogenation route are presented which cover examining hydrogenation efficiency and degree of hydrogenation. It has been found that there are side reactions at the interphase of the latex particles and within the polymer phase, which generate radicals to initiate the crosslinking of polymers in the latex form. Using radical scavengers did not show any evidence in helping to suppress the degree of gel formation.
Although there are methods developed to reduce the crosslinking, the aforementioned diimide route still encounters gel formation problem, especially when high hydrogenation conversion is achieved. Therefore, the resulting hydrogenated rubber mass is difficult to process or is unsuitable for further use because of its macroscopic three dimensional cross-linked structure.
U.S. Pat. No. 5,272,202 describes a process for the selective hydrogenation of the carbon-carbon double bonds of an unsaturated, nitrile-group-containing polymer with hydrogen in the presence of a hydrogenation catalyst being a palladium compound. In this process an aqueous emulsion of the unsaturated, nitrile-group-containing polymer is subjected to the hydrogenation and additionally an organic solvent capable of dissolving or swelling the polymer is used at a volume ratio of the aqueous emulsion to the organic solvent in a range of from 1:1 to 1:0.05. The aqueous emulsion is brought into contact with gaseous or dissolved hydrogen while maintaining an emulsified state.
U.S. Pat. No. 6,403,727 discloses a process for selectively hydrogenating C═C double bonds in polymers. Said process involves reacting the polymers with hydrogen in the presence of at least one hydrogenation catalyst selected from the salts and complex compounds of rhodium and/or ruthenium in an aqueous dispersion of the polymers which comprises up to 20% by volume of an organic solvent. The suitable rhodium containing catalysts are rhodium phosphine complexes of the formula Rh XmL3L4(L5)n wherein X is a halide, the anion of a carboxylic acid, acetylacetonate, aryl- or alkylsulfonate, hydride or the diphenyltriazine anion and L3, L4 and L5 independently are CO, olefins, cycloolefins, dibenzophosphol, benzonitrile, PR3 or R2P-A-PR2, m is 1 or 2 and n is 0, 1 or 2, with the proviso that at least one of L3, L4 or L5 is one of the above mentioned phosphorus-containing ligands of the formula PR3 or PR2-A-PR2, wherein R is alkyl, alkyloxy, cycloalkyl, cycloalkyloxy, aryl or aryloxy. U.S. Pat. No. 6,566,457 makes use of the same principal technology of hydrogenating a polymer in latex form in the presence of a ruthenium and/or rhodium containing catalyst in order to prepare graft polymers.
JP 2001-288212 A describes a further process for hydrogenating diene-based polymer latices. Latices of 2-chloro-1,3-butadiene (co)polymers are mixed with solutions or dispersions of catalysts in organic solvents which dissolve or swell the (co)polymers, and are then contacted with hydrogen. The catalysts used are the so-called Wilkinson-catalysts having the formula MeCla(P(C6H5)3)b wherein Me is a transition-metal, Cl is chlorine, b is an integer and equal to or bigger than 1 and a+b is an integer less than or equal to 6. In the Examples a latex of poly(2-chloro-1,3-butadiene) rubber having a Tg of −42° C. and an average number weight molecular weight Mn of 150.000 was added to a toluene solution containing RhCl(PPh3)3 and Ph3P, and hydrogenated at 100° C. and 5.0 MPa for 2 hours to give a hydrogenated polymer with Tg−54° C. and Mn=120.000.
In the Journal of Applied Polymer Science, Vol. 65, no. 4, 1997, 667-675, two processes for the selective hydrogenation of the C═C bonds in nitrile-butadiene rubber (“NBR”) emulsions are described, which are carried out in the presence of a number of RuCl2(PPh3)3 complex catalysts. One of the processes is carried out in a homogeneous system, in which an organic solvent, which can dissolve the NBR polymer and the catalyst and which is compatible with the emulsion, is used. The other process is carried out in a heterogeneous system, in which an organic solvent, which is capable of dissolving the catalyst and swelling the polymer particles but is not miscible with the aqueous emulsion phase, is used. Both processes can realize quantitative hydrogenation of the C═C double bonds with the help of a certain amount of organic solvent to dissolve or swell the polymers.
U.S. Pat. No. 6,696,518 teaches a process for selective hydrogenation of nonaromatic C═C and C≡C bonds in polymers with hydrogen in the presence of at least one hydrogenation catalyst comprising ruthenium and/or rhodium and at least one nonionic phosphorus compound capable of forming a coordinative compound with the transition metal wherein the hydrogenation catalyst is incorporated into the aqueous dispersion of the polymer without adding a solvent. Ru and/or Ru complexes or Ru and/or Ru salts are used as catalysts. Examples of preferred nonionic phosphorus compound are PR3 or R2P(O)xZ(O)yPR2 [R represents e.g. C1-10 alkyl, C4-12 cycloalkyl, C1-10 alkoxy, aryl(oxy) and F; Z is a bivalent hydrocarbon residue; x, y=0, 1]. For this special case, an acrylic acid-butadiene-styrene copolymer latex was prepared by radical polymerization of a mixture of monomers containing also ruthenium(III) tris-2,4-pentanedionate, which means the Ru salt was dispersed into monomer aqueous solution as the catalyst precursor before the polymerization. After having obtained the aqueous polymer dispersion, Bu3P was added to the latex. The system was stirred for 16 h at ambient temperature followed by hydrogenation at severe conditions for 30 hours at 150° C. and 280 bar. The catalyst was thereby synthesized in-situ, therefore no organic solvent was used to transport the catalyst. The hydrogenation is carried out in aqueous dispersions instead of in an organic medium, although the in-situ synthesized catalyst is oil-soluble. However, the operating procedure of U.S. Pat. No. 6,696,518, i.e. adding the catalyst precursor to the monomer mixture before the polymerization takes place, is associated with some problems, including that the catalyst precursor may have a negative effect on the polymerization and that some of the catalyst precursor might get deactivated during the polymerization.
In J. Molecular Catalysis Vol. 123, no. 1, 1997, 15-20 it is reported on the hydrogenation of polybutadiene (PBD), as well as polymers having styrene-butadiene repeating units (SBR) or having nitrile-butadiene repeating units (NBR) which are present in emulsions. Such hydrogenation is catalyzed by water-soluble rhodium complexes like e.g. [RhCl(HEXNa)2]2 (HEXNa=Ph2P—(CH2)5—CO2Na) and RhCl(TPPMS)3 (TPPMS=monosulphonated-triphenylphosphine). The process, however, is carried out in the presence of some organic solvent. Under the conditions employed the catalyst is extracted into the organic phase during reaction. This is attributed to the phase transfer properties of the complex which is rendered by the amphiphilic HEXNa ligand.
In Rubber Chemistry and Technology Vol. 68, no. 2, 1995, 281-286 it is described to use a water-soluble analog of the Wilkinson catalyst, i.e. RhCl(TPPMS)3 (where TPPMS represents monosulphonated-triphenylphosphine), for hydrogenation of nitrile rubber latex without any organic solvent. The hydrogenation occurs under mild conditions (1 atm of hydrogen pressure, 75° C.) without coagulation of the latex, and up to 60 percent hydrogenation can be achieved. It is recognized that the hydrogenation was accompanied by an increase in gel content of the latex.
JP 2003-126698 A discloses a process for hydrogenating unsaturated polymer latices using a water soluble catalyst containing group VIII metals or their compounds and hydrophilic organic phosphine ligands in the absence of organic solvents. In the process as disclosed in the Examples, 0.133 mmol (as Rh) chloro(1,5-cyclooctadiene)rhodium dimer was stirred with 0.372 mmol of P(C6H4SO3Na)3 to generate an aqueous solution of the complex catalyst. One part of such catalyst solution was mixed with five parts of butadiene rubber latex without organic solvent for hydrogenation. However, the highest degree of hydrogenation is only about 56% which is unsatisfactory for a scale-up to larger production volumes.
In the Journal of Molecular Catalysis, A: Chemical, Vol 231, no. 1-2, 2005, 93-101 it is reported to perform an aqueous phase hydrogenation of polybutadiene-1,4-block-poly(ethylene oxide) (PB-b-PEO) by using water-soluble Rh/TPPTS complexes. The hydrogenation can be successful only because the PB-b-PEO has water-soluble parts within its polymer chains. In such a hydrogenation system, mixed micelles are formed by mixing the amphiphilic PB-b-PEO with added cationic surfactant dodecyl trimethyl ammonium chloride (DTAC) and n-hexane. Hydrogenation conversion can go up to 100% after one hour catalyzed by Rh/TPPTS complexes ([Rh]=10 ppm or less in aqueous phase) generated in situ from RhCl33H2O and TPPTS under 80 to 100° C. and 20 bar of H2. Their recycling experiment showed that the catalytic activity of the anionic catalytic system, Rh/P(C6H4-m-SO3—)3, remained high in a consecutive run. The success of this hydrogenation system is mainly due to the fact that PB-b-PEO is an amphiphilic starting material. Therefore, the water-soluble catalyst works for systems which use an amphiphilic polymer material.
In US 2006/0211827 A1 a process for selectively hydrogenating nitrile-butadiene rubber latex without organic solvent is disclosed in which RhCl(PPh3)3 is used as catalyst and PPh3 is used as co-catalyst. The catalyst is added as pure material or in a small amount of organic solvent. This process can achieve high degrees of hydrogenation and does not show gel formation. However, this method requires long reaction times and high loading of the transition metal for synthesis of the catalyst.
In summary, several technical routes have been attempted to hydrogenate C═C double bonds in polymers in the form of latex, which include using hydrazine or a derivative of hydrazine as a reducing agent together with an oxidant like oxygen, air or hydroperoxide, directly using oil-soluble catalysts accompanied with a certain amount of organic solvents, and using catalysts containing water-soluble ligands. The hydrazine relevant route has encountered a significant gel formation problem, especially when a high hydrogenation conversion is required, and gel formation is not desired for post processing operations. In all the prior art references relevant to using oil-soluble catalysts, a certain amount of organic solvent is still required in order to achieve a reasonable hydrogenation rate. The route using water-soluble catalysts has also encountered significant difficulty in overcoming the crosslinking problem.
The present invention therefore had the object to provide a novel and improved process allowing the hydrogenation of a diene-based polymer present as an aqueous dispersion, i.e. as a latex, with a high degree of hydrogenation within short reaction times. The improved process shall eliminate the complicated catalyst synthesis operations so far necessary.